As users experience the convenience of wireless connectivity, they are demanding increasing support. WLANs are now offered by cafes, airports, hotels, businesses, residences, etc. Typical applications over wireless networks include video streaming, video conferencing, distance learning, etc. Because wireless bandwidth availability is restricted, quality of service (QoS) management is increasingly important in 802.11 networks.
The original 802.11 media access control (MAC) protocol was designed with two modes of communication for wireless stations (STAs). The first mode, Distributed Coordination Function (DCF), is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), sometimes referred to as “listen before talk.” A wireless station (STA) waits for a quiet period on the network and then begins to transmit data and detect collisions. The second mode, Point Coordination Function (PCF), supports time-sensitive traffic flows. Using PCF, wireless access points (APs) periodically send beacon frames to communicate network identification and management parameters specific to the wireless local area network (WLAN). Between beacon frames, PCF splits time into a contention period (CP) where the STAs implement a DCF protocol, and a contention-free period (CFP) where an AP coordinates access by the various STAs based on QoS requirements.
Because DCF and PCF do not differentiate between traffic types or sources, IEEE proposed enhancements to both coordination modes to facilitate QoS. These changes are intended to fulfill critical service requirements while maintaining backward-compatibility with current 802.11 standards.
Enhanced Distributed Channel Access (EDCA) introduces the concept of traffic categories (or access classes or traffic classes or ACs). Using EDCA, STAs try to send data after detecting that the wireless medium is idle for a set time period defined by the corresponding AC. A higher-priority AC will have a shorter wait time than a lower-priority AC. While no guarantees of service are provided, EDCA establishes a probabilistic priority mechanism to allocate bandwidth based on ACs.
The IEEE 802.11e EDCA standard provides QoS differentiation by grouping traffic into four ACs, i.e., voice (VO), video (VI), best effort (BE) and background (BK). Each transmission frame from the upper layers bears a priority value (0-7), which is passed down to the MAC layer. Based on the priority value, the transmission frames are mapped into the four ACs at the MAC layer. The VO AC has the highest priority; the VI AC has the second highest priority; the BE AC has the third highest priority; and the BK AC has the lowest priority. Each AC has its own transmission queue and its own set of AC-sensitive medium access parameters—the arbitration interframe space (AIFS) interval, contention window (CW, CWmin and CWmax), and transmission opportunity (TXOP). Traffic prioritization uses the medium access parameters to ensure that a higher priority AC has relatively more medium access opportunity than a lower priority AC.
Generally, in EDCA, AIFS is the time interval that a STA must sense the wireless medium to be idle before invoking a backoff mechanism or transmission. A higher priority AC uses a smaller AIFS interval. The contention window (CW, CWmin and CWmax) indicates the number of backoff time slots until the STA can attempt another transmission. The contention window is selected as a random backoff number of slots between 0 and CW. CW starts at CWmin. CW is essentially doubled every time a transmission fails until CW reaches its maximum value CWmax. Then, CW maintains this maximum value CWmax until the transmission exceeds a retry limit. A higher priority AC uses smaller CWmin and CWmax. A lower priority AC uses larger CWmin and CWmax. The TXOP indicates the maximum duration that an AC can be allowed to transmit frames after acquiring access to the medium. To save contention overhead, multiple transmission frames can be transmitted within one TXOP without additional contention, as long as the total transmission time does not exceed the TXOP duration.
To reduce the probability of two STAs colliding, because the two STAs cannot hear each other, the standard defines a virtual carrier sense mechanism. Before a STA initiates a transaction, the STA first transmits a short control frame called RTS (Request To Send), which includes the source address, the destination address and the duration of the upcoming transaction (i.e. the data frame and the respective ACK). Then, the destination STA responds (if the medium is free) with a responsive control frame called CTS (Clear to Send), which includes the same duration information. All STAs receiving either the RTS and/or the CTS set a virtual carrier sense indicator, i.e., the network allocation vector (NAV), for the given duration, and use the NAV together with the physical carrier sense when sensing the medium as idle or busy. This mechanism reduces the probability of a collision in the receiver area by a STA that is “hidden” from the transmitter STA to the short duration of the RTS transmission, because the STA hears the CTS and “reserves” the medium as busy until the end of the transaction. The duration information in the RTS also protects the transmitter area from collisions during the ACK from STAs that are out of range of the acknowledging STA. Due to the fact that the RTS and CTS are short, the mechanism reduces the overhead of collisions, since these transmission frames are recognized more quickly than if the whole data transmission frame was to be transmitted (assuming the data frame is bigger than RTS). The standard allows for short data transmission frames, i.e., those shorter than an RTS Threshold, to be transmitted without the RTS/CTS transaction.
With these medium access parameters, EDCA works in the following manner:
Before a transmitting STA can initiate any transmission, the transmitting STA must first sense the channel idle (physically and virtually) for at least an AIFS time interval. If the channel is idle after the initial AIFS interval, then the transmitting STA initiates an RTS transmission and awaits a CTS transmission from the receiving STA.
If a collision occurs during the RTS transmission or if CTS is not received, then the transmitting STA invokes a backoff procedure using a backoff counter to count down a random number of backoff time slots selected between 0 and CW (initially set to CWmin). The transmitting STA decrements the backoff counter by one as long as the channel is sensed to be idle. If the transmitting STA senses the channel to be busy at any time during the backoff procedure, the transmitting STA suspends its current backoff procedure and freezes its backoff counter until the channel is sensed to be idle for an AIFS interval again. Then, if the channel is still idle, the transmitting STA resumes decrementing its remaining backoff counter.
Once the backoff counter reaches zero, the transmitting STA initiates an RTS transmission and awaits a CTS transmission from the receiving STA. If a collision occurs during the RTS transmission or CTS is not received, then the transmitting STA invokes another backoff procedure, possibly increasing the size of CW. That is, as stated above, after each unsuccessful transmission, CW is essentially doubled until it reaches CWmax. After a successful transmission, CW returns to its default value of CWmin. During the transaction, the STA can initiate multiple frame transmissions without additional contention as long as the total transmission time does not exceed the TXOP duration.
The level of QoS control for each AC is determined by the combination of the medium access parameters and the number of competing STAs in the network. The default EDCA parameter values used by non-AP QoS stations (QSTAs) are identified in the table of FIG. 1. A TXOP_Limit value of 0 indicates that a single MAC service data unit (MSDU) or MAC protocol data unit (MPDU), in addition to a possible RTS/CTS exchange or CTS to itself, may be transmitted at any rate for each TXOP.
In a point-coordinated access mode, e.g., PCF, a single WLAN includes at least one AP in communication with one or more STAs. The combination of the single AP and its STAs is referred to as a “basic service set” or “BS S.” FIG. 1B illustrates an example BSS network 100, which includes two BSSs 105a and 105b (each generally referred to as a BSS 105), each coupled to a computer network 110 such as the wide area network commonly referred to as the Internet. In the example shown, the BSS 105a includes an AP 115a and three (3) STAs 120a. The BSS 105b includes an AP 115b and two (2) STAs 120b. Wireless communication by the STAs 120a of BSS 105a goes through the AP 115a. Wireless communication by the stations 120b of BSS 105b goes through the AP 115b. Since most corporate WLANs require access to a wired LAN for services (e.g., file servers, network printers, Internet links, etc.), corporate WLANs typically operate using a point-coordinated access mode.
In a distributed access mode, e.g., DCF, a group of STAs operate in a manner analogous to a peer-to-peer network, in which there is no AP and no single STA is required to function as the AP. The combination of STAs in the ad-hoc network is commonly referred to as an “independent basic service set,” “independent BSS” or “IBSS.” FIG. 1C illustrates an IBSS 150 having four (4) STAs 155. As shown, each STA 155 is capable of communicating directly or indirectly with the other STAs 155 of the IBSS 150. IBSS 150 is useful when quick and easy setup of a WLAN is desired, where connection to a wired network is not needed (e.g., where services may not be offered, such as in a hotel room, convention, center, airport, etc.), and/or where access to a wired network is barred (e.g., for consultants at a client site). A BSS 105, e.g., BSS 105a and/or BSS 105b of FIG. 1B, may include an IBSS 150.
It should be appreciated that a WLAN operating using infrastructure mode, ad-hoc mode or a combination of the two can be referred to as a BSS 105.
When two or more BSSs 105 are located proximate to one another and are operating over the same channel, link quality may deteriorate, e.g., due to contention among the overlapping BSSs and/or signal interference. Accordingly, it becomes difficult to guarantee QoS over WLANs, e.g., for real-time multimedia applications. For example, in FIG. 1, if the BSS 105a were located proximate to BSS 105b, then the BSSs 105a and 105b may interfere with each other.
Systems and methods are needed to improve link quality caused by overlapping BSSs 105. Example prior art references include:
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Pending Application SNInventorFiling DateU.S. Pat. No. 11/588,778ZhaoOct. 26, 2006